The present invention relates to an output circuit and a data transfer device which are used in a semiconductor integrated circuit.
Conventionally, a tri-state type output circuit and an open drain type output circuit have been employed separately. In a parallel processing system which is currently being developed by the present inventors, it is necessary to perform one-to-one transfer in which data is transferred from a data transfer circuit to one of a plurality of data reception circuits and broadcast transfer in which data is transferred from the data transfer circuit to all of a plurality of the data reception circuits. At the time of data transfer, the data reception circuits inform, by using a flag signal indicative of an internal state of each of the data reception circuits, the data transfer circuit whether or not data reception is possible. In the case where an output circuit of this flag signal is constituted by a tri-state type output circuit, the flag signal is connected to the data transfer circuit through each data reception circuit. At the time of broadcast transfer, the data transfer circuit takes a logical product of all the flag signals so as to judge whether or not data transfer is possible. On the other hand, in the case where an output circuit of the flag signal is constituted by an open drain type output circuit, the flag signals from the respective data reception circuits are used through wired connection but a pull-up resistance for pulling up and maintaining each flag signal at a sufficiently high level must be provided.
Thus, in the known data transfer device employing the tri-state type output circuits, the number of flag signals increases in response to an increase of the number of data reception circuits and the circuit for taking the logical product of the flag signals is also made complicated, thereby resulting in an excessively large circuit area. Meanwhile, since the number of data reception circuits has a tendency to increase in the future, the number of signal lines will be increased more and more and thus, a serious problem arises as the circuit area increases.
On the other hand, in the known data transfer device employing the open drain type output circuits, the pull-up resistance is used for pulling up and maintaining to a high level the signal obtained by wired connection of the flag signals. At this time, contradictory demands for reduction of the pull-up resistance so as to increase the operational speed and for increase of the pull-up resistance so as to reduce power consumption are made, and the pull-up resistance comes close to its limit in a high-speed integrated circuit of low power consumption.